As the core technology in the field of thermal power generation, the technology of turbo-generator unit has been developed for more than one hundred years, and in recent years, with the continuous increase in the fuel price and environmental protection pressure, the developed countries start to compete in developing high efficiency supercritical turbo-generator units. While constantly improving the operational parameters of the units to increase the cycle efficiency of the units, the double-reheat cycle unit has once again become one of the primary technical solutions for designing the next generation of high efficiency ultra-supercritical units.
At present, the coal-fired generating unit with the highest efficiency in the world is the 400 MW double-reheat ultra-supercritical unit in Denmark, with its steam parameters of 29 MPa/580° C./580° C./580° C. The cycle efficiency of the unit is as high as 47%, better than the cycle efficiency 41˜42% of the single-reheat supercritical unit and the cycle efficiency 44˜45% of the single-reheat ultra-supercritical unit.
However, with respect to the traditional single-reheat unit, if the double-reheat supercritical unit of 31.1 MPa/566° C./566° C./566° C. with an additional single reheat is compared with the single-reheat ultra-supercritical unit of 24.5 MPa/600° C./600° C. which only increases the steam temperature, the cycle efficiency of the former is merely improved by 0.5% more than that of the latter. While compared to the single-reheat (ultra) supercritical unit, the system of the double-reheat (ultra) supercritical unit is more complicated and has a significantly increased manufacturing cost. Therefore, since the 1990s, all the countries in the world generally tend to build single-reheat ultra-supercritical units of 600° C. class.
Illustrated in FIG. 1 is a schematic view of the conventional arrangement of a 1000 MW-class tower type boiler 1 and a double-reheat turbine unit 2 as in the prior art. An HP (High Pressure) cylinder 21, an IP (Intermediate Pressure) cylinder I 22, an IP cylinder II 23 and two LP (Low Pressure) cylinders 24 in the turbine unit 2 are in a single shaft arrangement in the turbine hall. Generally, a main steam pipe 31 extending from the outlet of a superheater 13 of a boiler 1 to the HP cylinder 21 has a single pipe length of about 160 meters; then, by way of a low temperature single-reheat steam pipe 32 with a single pipe length of about 180 meters, the steam exhausted from the HP cylinder 21 returns to the boiler 1; the high temperature single-reheat steam pipe 33 extending from the outlet of the single-reheater 12 to the IP cylinder I 22 has a length substantially the same as that of the low temperature single-reheat steam pipe 32, with a single pipe length of about 190 meters; subsequently, the low temperature double-reheat steam pipe 34, through which the steam exhausted from the IP cylinder I 22 returns to the boiler 1, has a single pipe length of about 180 meters; the high temperature double-reheat steam pipe 35 extending from the outlet of the double-reheater 16 to the IP cylinder II 23 has a length substantially the same as that of the low temperature double-reheat steam pipe 34, with a single pipe length of 190 meters. The main steam pipe 31, the high temperature single-reheat pipe 33 and the high temperature double-reheat pipe 35 need to be made of high temperature alloy steel of 600° C. class. According to various pipe design schemes, the main steam pipe and the reheat steam pipe also have pipes with a half capacity and with a quarter of capacity. As a result, the lengths of the high-pressure high-temperature pipes that actually need to be made of high temperature alloy steel will be multiplied.
Illustrated in FIG. 2 is a schematic view of the conventional arrangement of a 1000 MW-class tower type boiler 1 and a double-shaft double-reheat turbine unit 2 as in the prior art. The turbine unit 2 is composed of two shaftings which respectively have self-contained generators and are arranged in parallel in a conventional turbine hall. The first shafting 21 in the turbine unit 2 is composed of an HP cylinder 211 and an IP cylinder I 212, and has a self-contained generator; the second shafting 22 is composed of an IP cylinder II 221 and two LP cylinders 222, and has a self-contained generator. As to the double-reheat double-shaft turbo-generator unit as shown in FIG. 2, the arrangement scheme of its HTHP (high-temperature high-pressure) pipes is similar to that of the single-shaft double-reheat turbine unit as shown in FIG. 1, in which the main steam pipe 31, the high temperature single-reheat pipe 33 and the high temperature double-reheat pipe 35 are required to be made of high temperature alloy steel of 600° C. class, and their single pipe lengths are all about 160 meters.
At present, the unit prices of generating units are continuously increasing with the increase in the efficiency of the units. Especially, although the technology of the double-reheat unit has been well-developed, the boiler needs an additional stage of the reheater, the turbine needs an additional IP cylinder, and an additional stage of the reheat steam pipe is also needed. As a result, the system becomes complicated with more investment. As compared to the single-reheat unit, the acquired increment in the efficiency earning still fails to compensate the increase in the investment. For the traditionally designed double-reheat unit, first, the reheat steam shuttles between the boiler and the turbine hall, and especially, for the modern MW class large-scale (ultra) supercritical unit, the boiler is becoming increasingly taller, in combination with the structural factors such as the deaerator bay and the coal-bunker bay located between the turbine and the boiler, the average single pipe lengths of the steam pipes have reached 160 meters˜190 meters. On the one hand, the pipe of 600° C. class with a large diameter and a thick wall is high in cost; on the other hand, the increase in the system resistance of the reheat pipe results in a reduced work capacity of the turbine, such that the theoretical efficiency of the double-reheat system is partially diminished. Secondly, the double-reheat system greatly increases the quantity of steam stored in the system. As a result, the adjustment inertia of the unit is significantly increased, and thus the difficulty in controlling the unit is increased.
At the end of the 1990s, the Untied States, Japan and the European Union established the plans for developing the next generation of high efficiency ultra-supercritical units. All these plans aimed at the single-reheat and double-reheat units with nickel base superalloys as the base material. At present, the price of high temperature nickel base alloy steel of 700° C. class is more than 5 times of the price of high temperature alloy steel of 600° C. class. If such kind of material is used in a double-reheat 2×1000 MW ultra-supercritical unit, the investment for the four major pipes only will exceed RMB 2.5 billion. If the double-reheat cycle is adopted, the increase in the investment cost because of using nickel base alloy steel for building the boilers, turbines and HTHP steam pipes will generate no income on investment, considering the current fuel price.
Therefore, under the present technical conditions, material conditions and conventional design schemes, there is a contradiction between cost and benefit in improving the efficiency of the turbo-generator unit. This also has become a bottleneck that restricts the development of the next generation of high efficiency ultra-supercritical units. Furthermore, under the environmental protection pressure and the pressure of CO2 emission reduction, how to further “upgrade” the main facilities for thermal power generation—the subcritical and supercritical units—will become another issue in the development of the electric power industry.